In the performance of medical procedures, such as open heart surgery, isolated limb reperfusion, cerebral or neuro-perfusion, or the like, the patient may be supported by an extracorporeal blood circuit employing a heart flung machine. Isolated organs or limbs may also be separately supplied with a blood mixture fluid, which may include a portion of the oxygenated blood from such an extracorporeal blood circuit, with or without other constituents. In the specific case of open heart surgery, for example, the heart is isolated from the vascular system, and venous blood is diverted into the extracorporeal blood circuit where it is oxygenated, temperature-controlled and returned to the patient's arterial side. A separate circuit is established for supplying a blood mixture fluid to the heart as the surgery proceeds. The constituents of such a blood mixture fluid will depend upon the procedure. In some situations, the blood mixture fluid will include whole blood; in other uses, it may be a mixture of blood, plasma and/or platelets or blood mixed with other constituents, agents and or additives. For example, during open heart surgery, a patient's heart may be supplied with a cardioplegia solution to stop the heart. A different blood fluid mixture might be used to maintain the heart during surgery, and another for reperfusion after surgery.
When the blood mixture fluid is a cardioplegia solution, it functions to still the heart, lower the metabolic requirements of the heart, protect the heart during periods of ischemia, and, finally, prepare the heart for reperfusion at the end of the procedure. Operation of the extracorporeal blood circuit as well as the cardioplegia delivery is performed by a trained perfusionist under the direction of the cardiovascular surgeon. The principal elements of advanced cardioplegia solutions are blood, representing a small fraction diverted from the output of the heart/lung machine, combined with a crystalloid solution. The crystalloid solution is a premixed aqueous saline or glucose solution, metabolic substrates to feed the heart tissue, and may also contain buffers, special medications and additives to protect and preserve the heart during periods of ischemia and to prepare the heart for reperfusion with the normal blood supply. In addition, a minor but critical amount of potassium solution is added to the cardioplegic flow to still the heart.
Depending upon the requirements of the particular surgery, the cardioplegia solution may be cooled or warmed, and may be delivered in antegrade fashion to the aortic root or coronary ostia, or in a retrograde mode to the coronary sinus. The mode of delivery and composition of the cardioplegia solution may vary as the surgery proceeds, and are subject to the clinical judgment of the individual surgeon.
A typical cardioplegia delivery system employs two tubes routed through a single rotary peristaltic pump to forward both the separate blood and crystalloid solutions to a Y combining the two into a single flow. The ratio between the blood and crystalloid solution is determined simply by the relative diameters of the tubing carrying the two solutions, since each is mounted on the same rotary peristaltic mechanism and thus is forwarded by the same action. The tubing is usually provided in a 4:1 ratio of blood to crystalloid cross-sectional flow area, so that the rotary peristaltic pump is delivering blood and crystalloid to the delivery line in a ratio of approximately 4:1. Potassium is typically provided to the delivery line upstream of the pump from two alternate crystalloid solutions containing potassium, one having a relatively high concentration of potassium to stop the heart, the other a lower concentration of potassium sufficient to maintain the heart in the arrested state. The surgeon selects between the two sources as monitoring of the patient's condition indicates. The higher potassium concentration is utilized to arrest the heart, while the lower is used to maintain the stilled condition. The clinical team must provide sufficient potassium in the cardioplegia solution to establish the stilled condition of the heart and to maintain it during the procedure, while avoiding the risks associated with hyperkalemia and hemodilution which may result from excessive cardioplegia solution delivery.
Existing systems for delivery of cardioplegia are characterized by poor adaptability to varying requirements which the surgeon in charge may place upon the system as to parameters or characteristics of the cardioplegia fluid, including, for example, flow rate, pressure, ratios of blood to crystalloid, solution additives, temperature, and flow direction through the heart. For example, control of the ratio of blood to crystalloid solutions is not possible with present systems because the ratio is fixed by the size of the tubing routed through the peristaltic pump. Some of these parameters or characteristics have been controlled to a limited extent in prior systems. The control, if any, has been independently effectuated at separate instrumentation which has been interconnected with tubing to form a complete cardioplegia delivery system. The systems have particularly poor control over the cardioplegia delivery at low flow rates. Moreover, the shearing forces to which the blood in the cardioplegia line is subjected by peristaltic pump action risks damage to the blood. Furthermore, existing systems depend primarily on controlling the flow rate through manual adjustments in order to maintain a sufficient pressure to force the cardioplegia fluid through the vascular system of the heart while avoiding excessive pressure which might rupture the vessels or heart tissue. Also, the requirements are different for antegrade perfusion and for retrograde perfusion.
In our co-pending patent application filed on May 26, 1993, Ser. No. 08/067,683, which is incorporated herein by reference as if set forth herein, a cardioplegia system is provided for delivering cardioplegia solutions to a heart during open-heart surgery. The system cooperates with an extra-corporeal blood circuit employing a heart/lung machine. The system includes a conduit for diverting a portion of the blood flow from the heart/lung machine to the cardioplegia delivery line. A heat exchanger controls fluid temperature in the cardioplegia delivery line. A first pump combines blood from the conduit with a second fluid and delivers the combined fluid flow into the delivery line leading into the heat exchanger. A second pump is provided for delivery of a third fluid, typically the arrest agent, into the delivery line downstream from the first pump. The second pump has a flow rate less than about ten percent (10%) of the flow rate of the combined output of the first pump. Control means are included for adjusting the ratio of blood and second fluids which is delivered by the first pump and for adjusting the total volumetric rate of flow from the pump. Preferably, the volumetric flow rate of the first, second and third fluids are maintained at its desired percentage relative to each other. A third pump is provided for delivery of a fourth fluid, typically an additive, in combination with the output of the first and second pumps. Control means are provided for automatically controlling the output of the third pump in proportion to the variable output of the first pump.
In giving cardioplegia solutions for the heart, either for delivering an arrest agent which stops the heart for surgery or other additives which help take care of the heart, it is important to have adequate perfusion of all regions of the heart to ensure adequate preservation of the heart muscle. Delivery of an adequate flow rate does not guarantee that all regions of the heart are adequately perfused. In fact, most users increase the flow rate until an operating pressure is achieved and then attempt to operate at or near the same flow rate so that the pressure is likely to stay within a limited range. It is extremely difficult to control the flow rate in order to keep the pressure relatively constant because it is a continually dynamic system. Furthermore, it is not uncommon for the surgeon to pick up the heart or to move it and cause high resistance at the tip of the cannula, which increases the pressure in a dramatic fashion. If that is not immediately observed by the surgeon, the heart can be ruptured or other significant problems can arise.
One reason that the surgeon attempts to deliver the fluid at a pressure within a certain range is the fact that all of the flow directed towards the heart does not go through the blood vessels for nourishing the heart muscle tissues. For example, the veins which return the blood from the heart capillary beds to the coronary sinus form an intercommunicating network which when cardioplegia solution is delivered in the retrograde direction through the coronary sinus may route a percentage of the solution into veins which actually shunt the blood to the right atrium or into the left ventricle, bypassing the heart tissue. There may also be leakage around a balloon which is used to seal off the blood vessel, typically the coronary sinus, to which the fluid is being delivered. In the antegrade direction, occasionally the aortic valve may be defective or become incompetent due to manipulation of the heart by the surgeon. In either situation, the cardioplegia solution is delivered into the left ventricle rather than the coronary arteries. Therefore, as a result, one cannot be sure that upon delivering a certain amount of fluid, an adequate amount of the fluid will actually go through the tissues of the heart. Surgeons and their perfusionists use the fluid delivery pressure as a measure to indicate adequate delivery to the heart tissues. The particular flow rate necessary to achieve a desired pressure will depend upon the system leakage and the condition of the patient. The placement and seal of the delivery catheter into the aortic root or coronary ostia for antegrade infusion, or into the coronary sinus for retrograde infusion, along with the condition of the patient's blood vessels will also affect whether the flow provided actually moves through the heart tissue. Flow which leaks out of the system will not be useful for maintaining the healthy heart tissue and may contribute to hemodilution and hyperkalemia of the patient.
With most prior blood mixture fluid delivery systems, the control of fluid delivery, as, for example, cardioplegia delivery, is based on the perfusionist's ability to observe changes in pressure and manually adjust the rate of delivery, accordingly. Recently improved devices with an upper pressure unit address only part of the problem. One recently developed device proposes automatically reducing flow rate when an upper pressure limit is reached. Flow rate control returns whenever the pressure is below the set limit. This proposal only addresses part of the inadequacies of prior cardioplegia delivery systems. The true goal should be to provide adequate flow through blood vessels supplying the tissues of the organ or limb. In the case of myocardial surgery, the goal is to have adequate flow to the heart tissues, particularly to the myocardium, at a pressure sufficient to ensure that the principal pathways are adequately perfused but not excessive as to avoid damage to the heart. Excessive pressures may result in rupture of blood vessels, such as the coronary sinus, and should be avoided. However, institution of an upper pressure limit, by itself, would not address the question of adequate delivery through the blood vessels at all times. Thus, safe and adequate delivery of blood mixture fluid to the intended tissue, such as delivery of cardioplegia solution through the heart, is dependent upon the pressure of delivery as well as the flow rate.